CO 2 -induced chloroplast movement in one cell-layer moss leaves

CO 2 -induced chloroplast movement was reported in the monograph by Gustav Senn in 1908: unilateral CO 2 supply to the one cell-layered moss leaves induced the positively CO 2 -tactic periclinal arrangement of chloroplasts. However, from the modern criteria, several experimental settings are unacceptable. Here, using a model moss plant Physcomitrium patens , we examined basic features of chloroplast CO 2 -tactic relocation with a modernized experimental system. The CO 2 relocation was light-dependent and especially the CO 2 relocation in red light was substantially dependent on photosynthetic activity. Between the cytoskeletons responsible for chloroplast movement of P. patens , the microfilament mainly worked for CO 2 relocation, but the microtubule-based movement was insensitive to CO 2 . The CO 2 relocation was induced not only by air with and without CO 2 but also by the more realistic difference in CO 2 concentration between the two sides. In the leaves placed on the surface of a gel sheet, chloroplasts avoided the gel side and positioned in the air facing surface. This was also shown to be photosynthesis dependent. Based on these observations, we propose a working hypothesis that the threshold light intensity between the light-accumulation and -avoidance responses of the photorelocation would be increased by CO 2 , resulting in the CO 2 -tactic relocation of chloroplast.